Hepatitis C (HCV) is prevalent in 80-90% of injection drug users in Baltimore, and drug use in the most important risk factor for HCV infections in the United States. However, the factors that increase the incidence of HCV infections in drug users and the long-term implications of HCV in this population are poorly understood. Two studies of hemophiliacs have suggested that human immunodeficiency virus (HIV) infection accelerates the progression of HCV- induced liver disease. In addition, HIV infection may enhance heterosexual and perinatal transmission of HCV- induced liver disease. In addition, HIV infection may enhance heterosexual and perinatal transmission of HCV, presumably because of higher quantities of HCV in HIV- coinfected persons. While drug users have high rates of both HCV and HIV, prospective research of this population has been limited by low rates of follow-up. However, we have followed a cohort of two thousand injection drug users since 1988 and have a well-characterized repository of sera and behavioral data on these participants. in this investigation, we propose using these resources to investigate whether HIV infection or behavioral factors increases the rate o acquisition of HCV and to assess if HIV increases the progression of liver disease in HCV- infected members of the cohort. Using newly-developed technology to measure the amount of HCV in sera, we will also explore the hypothesis that HIV infection increases the quantity of HCV.